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If both nodes are restarted at the same time, e.g. after a power failure, it is likely that manual intervention will be required, to clean up the data directories.
Log Files
On our IdPs, the terracotta server log is in /usr/local/shibboleth-idp/cluster/server/logs/terracotta-server.log. Normally, this will contain regular statistics logs of memory usage, but also exceptional events, including problems communicating with clients. We also capture the server standard output/error in /usr/local/terracotta/logs/terracotta.log.
The terracotta client log is in /usr/local/shibboleth-idp/cluster/client/logs-127.0.0.1/terracotta-client.log.